Tree encroachment impacts on seed predator selection and seedling establishment in degraded pine woodlands
While much is known about the impact of tree encroachment on flammability in degraded pine woodlands, little is known about how encroachment is impacting other important ecosystem functions. We investigated how the availability of seed from four encroaching tree species and the presence of a midstorey and litter layer affect seed predator selection. Additionally, we investigated how seed predators, the midstorey, overstorey basal area, substrate availability, and vegetation cover affect germination for a foundational species (Pinus palustris) compared to an encroaching species (Pinus taeda). Location: Sandhills Ecoregion, NC, USA (35°3′34.6932″ N, 79°22′22.0872″ W). Methods: We measured seed depredation of Pinus palustris, Pinus taeda, Liquidambar styraciflua, Acer rubrum, and Quercus nigra in cafeteria trials. Each trial was held within a 2 × 2 factorial involving vertebrate seed predator exclusion and midstorey and litter layer removal across a gradient of overstorey basal area (6–25 m2). Additionally, we measured Pinus palustris and Pinus taeda germination within each treatment and correlated germinant density to substrate and understorey vegetation cover. Results: Granivory generally varied inversely with seed size, with small-seeded Liquidambar styraciflua experiencing the highest (27%) and large-seeded Quercus nigra (7%) and Acer rubrum (6%) the lowest depredation pressure. Pinus palustris and Pinus taeda germinant density was significantly highest where vertebrate seed predators were excluded and the midstorey and litter layer were removed. For both pine species, this result corresponded with a significant positive association with mineral soil and negative associations with hardwood and pine litter where vertebrate predators were excluded. Basal area did not affect granivory or germination for any species. Conclusions: Our results demonstrate that granivores did not select Pinus palustris, and that large-seeded species encroachment was less inhibited by seed predators. Pinus palustris and Pinus taeda are depredated at comparable rates and germinate best under similar understorey conditions.